The present invention relates to overvoltage protection devices and, more specifically, to a device for protecting integrated circuits, and the like, from static discharges and associated fields comprising a housing of an electrically insulating material defining a chamber; an ionizable gas disposed within the chamber, and, a plurality of electrical leads carried by the housing in non-contacting relationship with each other communicating with the chamber and the gas on one end and in electrical contact with respect to each one of the electrical leads of the circuit to be protected on the other end.
The necessity to protect electrical and electronic circuits from overvoltages has long been recognized. As electronic circuits have become smaller and smaller, however, the source and power of what constitutes an "overvoltage" has rapidly diminished. Whereas, a lightning strike was the order of magnitude under consideration, more recently, the size of integrated circuits, for example, has diminished to a point where a single electrostatic discharge from walking across the carpet can be catastrophic.
It is widely known within the electronics industry that damage caused by electrostatic discharge is the primary source of unpredictable failure of integrated circuits. Triboelectric charges are produced anytime two surfaces are separated and if one or more of the surfaces is a non-conductor, then a static electric charge is produced. This is a natural phenomena and only becomes a problem if the static charge is allowed to discharge or induce a charge into the integrated circuit. Such electrostatic discharge events can occur very pervasively to a potential of several thousands of volts. The discharge occurs very rapidly and the usual failure or degragation is caused by the gasification of metal within the device resulting in the gasified metal becoming deposited along the trace of the discharge path. Following each such electrostatic discharge event that an integrated circuit is exposed to, the damage may be instantly catastrophic or, on the other hand, the device may just have its useful life put into question. Oftentimes, following such an electrostatic discharge event, the integrated circuit does not totally fail but, rather, remains totally operable yet containing a latent defect that will result in premature failure. Such events can also alter the operating characteristics of the integrated circuit resulting in unsatisfactory and often unpredictable operation.
In addition to the above-described metallic gasification problem resulting from the passage of the high voltage charge through the circuit, high voltage discharge in proximity to the device can produce high electromotive force fields that can induce damaging voltages into the circuit.
As stated above, overvoltage protection circuits are not new in the art. Some examples of various devices known in the art can be seen with reference to the following U.S. Pat. Nos. 1,656,956; 2,620,453; 2,967,256; 3,934,175; 4,037,139; 4,105,929; 4,207,603; 4,293,887; 4,318,149; and 4,438,477. In the field of integrated circuits and other similar, miniature, low voltage devices, primary emphasis has been on protection of the device during manufacture, transportation, and assembly. Typical examples of such devices can be seen with reference to U.S. Pats. Nos. 3,638,071 and 4,084,210. Primarily the intent is to electrically interconnect all the leads of the device during shipment and assembly when the device is inactive. The protective means are then removed prior to actual use of the inetegrated circuit. There has been some attempts to provide protection for such electrical devices on a more permanent nature. For an example as shown in U.S. Pat. No. 3,742,420, wherein the wafer of metallic oxide varistor material has the leads of an electrical device pass therethrough. At normal, low operating voltages, very little current flows through the wafer material between the leads of the device. At higher voltages, however, the leads are purportedly electrically inter-connected. Such an approach might work for some devices where the slight current flow between the various leads does not effect normal operation of the device. With more sophisticated circuits, however, complete isolation of the leads from one another is necessary to proper operation of the device. Such an approach to protection of the device, would, therefore, not work. A similar approach is shown in U.S. Pat. No. 4,458,291 wherein the intent is only to provide a shunting path for electrostatic charges applied to the device when it is picked up by the edge. No general protection as discussed above is provided. Additionally, as is well known by those skilled in the art, normal assembly of integrated circuits is subjected to procedures such as the use of conductive and/or anti-static materials in the workplace during manufacture, shipping, at handling stations and during assembly and testing. Protective methods include grounding the worker by use of conductive wrist straps and heel straps. In some work areas, ionized air is directed over the work area to neutralize charges. Non-conductors, particularly plastics, are avoided in the work areas and, in many instances, where non-conductors must be used, they are coated with anti-static solutions.
In an attempt to control electrostatic discharge within the design of integrated circuits themselves, many advances have been made. For example, the geometric design of the circuit layout has been improved to avoid sharp angles between potential discharge paths and insulated material added at similar high risk locations within the device. Other methods used to improve immunity to electrostatic discharge damage include the adding of resistors, varistors, zenderdiodes and similar components that will impede or redirect the electrostatic discharge energy away from the critical elements of the device.
Wherefore, it is the main object of the present invention to provide integrated circuits and the like with protection against electrostatic discharge damage, regardless of polarity of the charge or pins carrying the charge.
It is an additional object of the present invention to provide a shield that will also protect the integrated circuit from induced voltages and from fields produced while the protective device is active during an electrostatic discharge event and otherwise.